Field of the invention
The present invention relates to a surface light source device and a prism sheet.
Description of the Related Art
In recent years, there has been a growing demand for portable liquid crystal display devices, such as notebook-size personal computers and tablet terminals. In addition, as a surface light source device serving as the backlight of a liquid crystal display device, there is a requirement for a surface light source device capable of switching the distribution of planar illuminating light from the liquid crystal display device according to a user's purpose of use, for example, an application in Which the user uses the liquid crystal display device for business purposes.
As a surface light source device used to switch a light distribution in two steps according to a user's purpose of use, there has been proposed, for example, a surface light source device comprising: a first light source and a second light source; a light guide plate disposed between the first light source and the second light source, the plate including a plurality of lines of prisms extending parallel to one another in the direction of light from the first light source and the second light source on a light-emitting surface and a plurality of concave lines on a light-reflecting surface opposite to the light-emitting surface; and a prism sheet which is arranged on the light-emitting surface of the light guide plate and through which light from the light guide plate is input (see Japanese Patent No. 5254658).
In the surface light source device of Japanese Patent No. 5254658, light from the light guide plate is emitted from the prism sheet by the prisms of the prism sheet as light having strong directivity in the normal direction of a prism sheet emitting surface when the first light source is turned on. Accordingly, the distribution of the luminance of the light emitted from the prism sheet with respect to an outgoing angle is formed into a distribution having one peak at an outgoing angle of approximately 0° when the first light source is turned on. Consequently, only the user positioned in the normal direction of the surface light source device can visually recognize the display of equipment in which the surface light source device is built.
When the first light source and the second light source are simultaneously turned on, the distribution of synthetic light emitted through the prism sheet is formed into a light distribution having a wide outgoing angle. Accordingly, the surface light source device of Japanese Patent No. 5254658 allows the distribution of light emitted from the prism sheet to be switched in two steps by changing the lighting mode of the light sources according to a user's purpose of use.
If the first light source and the second light source are simultaneously turned on in the surface light source device of Japanese Patent No. 5254658, however, in order to obtain a light distribution having a wide outgoing angle, the light distribution is formed into a distribution having three peaks which are local maximum points and bottoms which are local minimum points among adjacent peaks. Accordingly, portions of the surface light source device corresponding to the bottoms are recognized as dark-line portions on a display screen by a user who views a liquid crystal display device mounted with the surface light source device. The surface light source device may therefore cause a degradation in quality that a surface light source has to have.